1. Technical Field
The present invention generally relates to an electronic circuit for an energy storage device management system. More particularly, the present invention is directed to an electronic circuit for efficiently and accurately measuring individual voltages in a series connected electrochemical energy storage device which may be utilized with electric and hybrid vehicles.
2. Discussion
In order to commercialize electric and hybrid vehicles on a widespread basis, the energy storage devices or batteries, which are the most expensive component of the vehicle, must operate reliably through the life of the vehicle. In the typical configuration the batteries are formed from a stack of series connected electrochemical cells.
A common requirement for large stacks of electrochemical cells used in electric and hybrid vehicles, particularly in advanced applications such as lead acid, Li-Ion or NiMH battery packs, is the need to measure individual or groups of cell voltages almost simultaneously. In practice, this means the measurements should be taken within a time window of a few milliseconds.
With reference to FIG. 1, a common technique known within the prior art accomplishes voltage measurement through the use of a plurality of resistive divider circuits. More specifically, FIG. 1 shows an exemplary battery pack 10 having forty-eight energy storage cells B1 through B48 connected in series. A resistive voltage divider circuit 12 is connected between the positive terminal 16 of battery cells B2 through B48 and a common ground node 14. The discrete resistances R.sub.1, R.sub.2, . . . , R.sub.n are selected such that the output potentials V.sub.m1, V.sub.m2, . . . , V.sub.mn fall below a certain voltage limit, for example 4 volts, suitable for input to a multiplexer and AID converter. The voltage signals from each resistive divider circuit 12 can then be sampled and digitally processed. The actual nodal voltages V.sub.1, V.sub.2, V.sub.3, . . . , V.sub.48 become increasingly higher towards the top of the battery pack 10, such that in general: ##EQU1##
The voltage across each cell segment V.sub.B1, V.sub.B2, . . . , V.sub.B48 is then computed as the difference between the nodal voltages measured on either side of the cell according to the formula: EQU V.sub.Bn =V.sub.n -V.sub.n-1
For example, the voltage V.sub.B3 of cell B3 is measured by taking the difference between V.sub.3 and V.sub.2 provided by the respective voltage divider circuits 12.
The principal problem with this technique of voltage measurement is that a small error in measuring the nodal voltages V.sub.n translates into a large relative error in the measurement of segment voltages V.sub.Bn. These errors increase as the nodal voltages V.sub.n become increasingly larger towards the top or higher potential cells of the battery pack 10. For example, suppose: ##EQU2## EQU V.sub.n48 =V.sub.48 .multidot.k.sub.48 =4V,V.sub.48 =192V, EQU V.sub.n47 =V.sub.47 .multidot.k.sub.47 =4V,V.sub.47 =188V, EQU .thrfore.V.sub.B48 =V.sub.48 -V.sub.47 =4V.
If k.sub.48 is in error by .epsilon.=1%, and k.sub.47 is in error by .epsilon.=-1%, measurements of the nodal voltages indicate: EQU V.sub.48 =193.92V; V.sub.47 =186.12V EQU V.sub.B49 =7.8V., error=95%
Thus, the measurement error associated with this network of resistive divider circuits 12 and measurement technique could be in excess of 95%.
Furthermore, this error is nonuniformly distributed between the cell segments varying from a maximum of 2.epsilon. percent at the bottom to a maximum of 2n.times..epsilon. percent at the top of the battery pack 10. The latter renders this approach useless in applications where comparison of the cell segment voltages are used for diagnostics or corrective actions such as in cell balancing. Lastly, this conventional resistance network continues draining the cells of the battery pack 10 even when the resistance network is not in use.
While not specifically shown, a matrix of electromechanical relays can also be used for selectively switching across the cell segments of the battery pack. This approach results in slow measurement of cell voltages and is therefore not suitable for modern applications. In addition, such a relay based device also becomes too bulky and heavy for use with an electric or hybrid vehicle. Higher speed and accuracy can be achieved using a separate isolation amplifier for each battery segment, but this approach results in a relatively large and expensive system.
Accordingly, it is desirable to provide an electronic circuit for overcoming the disadvantages known within the prior art. It is also desirable to provide an electronic circuit which allows for a high degree of accuracy when measuring both the lowest potential cell voltages and the highest potential cell voltages. Moreover, it is desirable to provide a highly efficient electronic circuit which minimizes any loss within the circuit. Finally, it is desirable to provide an electronic circuit with various switched components to prevent the leakage of current from the energy storage device when the circuit is not being used.